My thoughts: there was a mechanic in Escape Velocity (IIRC), where if one ran out of fuel, one could make a hyperspace call for help if one had the credits, eventually getting a fuel tanker sent your way. If you were out of credits, you died, with an explanation about drifting through space because you ran out of fuel and couldn't afford a tanker (i.e., telling players why their choice didn't work- this meant that from then on, players kept a minimal balance of credits to cover that dire necessity).
This is both realistic and a good way to prevent this kind of feature from becoming a major game-balance hassle.
If players have a billion credits and want to fly their giant fleet across the wide empty spaces of the galaxy, calling for fuel tankers at phenomenal costs (say, raise the price depending on how far you are from a solar system) ... I'd honestly say that it's both more realistic and more likely to be Fun to let them, rather than force them to build the All-Tanker Fleet. If the price scaling was really reasonable, it'd also be a gentle landing pad for newbies figuring out the mechanics.
[EDIT]Also... since I know that we'll all want to mod it in (and it'll be popular, because it will provide a gentler mechanic) I'd strongly suggest having a Hull Mod that generates Fuel over time. If Fuel's handled as a float internally, then Fuel generation / day can be helpful. I'd also suggest having a Hull Mod that allows trading Fuel for Supplies over time. Both would eat a medium-largish OP amount and be an effect that got more efficient with larger hulls.
This would, if set up correctly, allow for fleets with very long-range traveling capabilities, even indefinite ones, in exchange for significant reduction of their combat effectiveness. Sounds like an exploration fleet to me. The something-for-nothing can be explained away by lore- "Giant p-space interstitial meshes project Infernium-capturing field effects, allowing fleets to gather the minute amounts of Infernium that drift between the stars"... or whatnot.[/EDIT]