Those mecha need starships if they actually want to go anywhere interstellar, so you had better believe that this galaxy teeming with trillions of men across billions of stars produces and operates vast fleets of ships to transport those mechs where they need to go.
By now interstellar travel is a mature technology and that means starship designs are also somewhat standardized. Most ships carry auxiliary craft, in addition to any mecha, and they are still referred to as "boats" as a generic collective out of centuries of naval tradition dating before the Coming of the Azure Flames. Being that Space Is Big, the size of these vessels can approach or exceed that of the smaller orbital stations, yet with automation being so routinely applied even a ship a quarter-mile long can be fully crewed with a compliment small enough for the captain to know every crewman by name. Artificial gravity generation is cheap enough to be used for merchant freighters and pleasure craft. Hulls are routinely built to withstand micrometer hits and electromagnetic charges that are commonplace in the sea of stars.
It is out of convenience that all of the nations of Mankind agreed to a common classfication by treaty in the Court of Stars, one promoted by the Church as an act of good will and common sense. This includes rank structures for military and merchant fleets, and some conventions on symbols designating such. Whether you're in the Thousand Stars of the Dragon, or the Free Canton Worlds, when the destroyer on patrol calls you to heave to you know that there's a Captain in command and he'll clearly stand out as such by his uniform. How the ship, and the men, appear and operate vary significantly from one domain to the next, but one can count on that ship being a destroyer and its master being a Captain.
The basic convention is thus:
- Scout: The smallest starship class. Its master, while called "Captain" out of centuries of tradition, is often a Lieutenant and this is either a first command for a fresh officer in a formal military or a career-making command for a dedicated scout service or similar organization (couriers, trouble-shooters, or something else focused around squad-sized teams). Boats are few, but they do have them; these are often shuttles for when no hangars or berths are available. Even civilian ones are armed, albeit for point-defense against debris and pirates, and some are reconfigured for use as a fast-attack vessel in the tradition of 20th century torpedo boats or as an anti-fighter corvette.
- Cruiser: The smallest ship in a flotilla or fleet, and often the command of those who prove themselves in a scout, this is a ship class that splits into two subtypes: corvettes and destroyers. Corvettes are fast and manueverable, making them commmonly used as a mobile external adjunct to a battleship's defenses, usually in an anti-fighter capacity but sometimes in a more specialized role such as electronic warfare. Destroyers go the other way, focusing on firepower outsized for their class, as a way of bringing fire to bear when the line's capital ships cannot do so themselves. The last common variant is the carrier; larger than the other two, they are a homeship to a fighter or mech squadron--sometimes a wing--and otherwise impotent as they usually only have point-defense weapons- this frame is also often configured to be used for hospital and fleet tender functions.
- Battleship: The ship of the line. Most are exactly what you expect; they are massive vessels with hull plating, shielding, and arms more than capable of reducing targets to ruin or worse. A few variants swap this to be a massive mega-carrier instead, housing thousands of fighters or mechs as if they were a hive, but most fleets eschew this option; battleships often have a squadron aboard for Combat Space Patrol duties, as they can and do operate independently on a regular basis and thus need to perform such things on their own. Configurations within a fleet vary little, but flagships can and often are the Ace Customs of fleet warfare- sometimes even wholly original designs, meant as testbeds for future fleet-wide systems.
- Fortresses: While most orbital installations are meant to be more-or-less static, some are actually ships capable of travel; these are rarely used in fleet actions on the attack, and are often involved only on the defense, but enough incidents--inspired by historical precedent from the centuries before--have insured that fortresses have had their place in the naval annals. They are a feared presence, enough to force fleet admirals to deal with them even if they are not actively involved in a battle, as the arms and auxiliaries at their disposal makes them a fleet unto themselves. They are the Super Robots of fleet warfare, even though most are built out of massive asteroids or small moons and not just built in emulation of them.
Merchant vessels rarely go bigger than a cruiser, as it's more profitable to go with a small, but powerful ship and use that to tow bulk containers or passenger modules from place to place in the model of a train. Since merchant traffic routinely sticks to a known and well-traveled route and most destinations have an orbital station, merchant ships rarely bother with trans-atmospheric capabilities or boats since they can use the elevator or a shuttle at the station- even on most fringe worlds such as Garmil's Gate.
Most organizations have some military naval capacity, as Space Is Big and the noble families can't be everywhere, and you only find unarmed ships in space that is well-patrolled (and densely-patrolled) space. Piracy happens, but it's rarely without a shot fired, especially if the pirates are barbarian reavers or otherwise under the influence of an enemy of Civilization. Pirates, and some other groups, use flagships not only as supreme command vessels, but as a mobile base in emulation of the Fortresses; they use mobility as a security layer, forever roaming their domain, and live at sea permanently instead of spending most of their days on a planet or station- visiting static locations only as part of their tour or when need requires it.
Next time, the planets of the galaxy.