low crew capacity which affects the ground support package
When i was reading the wiki on GSP it says each ship with the mod supports 100 marines, not just its max crew, is that not the case?
Ground Support Package basically gives you a bonus to your marine strength equal to whichever is smaller between the number of marines you have, or the value of GSP itself (100, I believe). In other words, if you have 100 marines or fewer in your fleet, GSP doubles your strength, but above 100 marines, it only adds 100. The max crew supported by the SHIP it's on doesn't matter.
Ship's cute! I reckon you could spin it with an upsized, more Starsector-fitting sprite and give it a regular modular weapons package. I'm not too hot on Halo content, but I'd be inclined to give a Pelican replica the Manoeuvring Jets system instead of flares, to imply the vectoring engines on the ship. But up to you.
I'm too out of practise with ship modding to spot any out-of-whack stats there... The only thing I'd suggest is upping the burn speed. For a frigate size, burn 9 is usually reserved for civilian ships. Also I'd recommend making at least one of the weapon mounts modular - ships with all built-in weapons take some fun out of Starsector, because a big part of the game is the ability to refit your ships' weapons (among other equipment).
0.5 fuel/ly is a good move, I'd do absolutely the same. Does the maximum of 20 passengers come from canonical information on the Pelican? I'd be inclined to give it a toned-down GSP, maybe one that only increases the marine strength by a max of 20 or 30... +100 on a sub-frigate seems pretty powerful, though you do need to use it in conjunction with other troop transports to make it worthwhile I suppose.