Frags are way better than plasmas once you get the hang of nade placement, don't know what you're saying, most of the kids I see with plasmas in inf slayer are just hoping for a lucky stick before dying
Shielding isn't that great but mobility is also unnecessary for small infinity slayer maps since default sprint lasts quite a while anyway. So usually mobility if you have to pick one of the two. Otherwise consider dexterity/resupply/explosives/ammo/ordnance priority for inf slayer
Obviously there's always going to be people trying to abuse the system to gain an edge; let's be honest even when I've played team slayer since h2 sometimes a teammates on the other team and you just get extra kills off them
Fuel rod is way better since rockets aren't as good as in reach and h3 and incineration has barely any ammo and takes no skill as long as you're vaguely aiming