IG are a great army to ally in, as they can take dirt cheap HQ and troops. They also have most of their good stuff in squadrons/platoons, meaning you can take most of an army, if you are clever, all while only using your ally slots.

For the HQ, look hard at a company command squad (CCS), which is BS4 and can take a heavy weapon or up to four specials. either sit it next to a quad gun to shoot BS4 lascannon/autocanno love, or give 'em four meltas/plasmas and drop 'em out of a valkyrie.

For troops, you can buy in for as little as a basic penal legion squad, which while unimpressive at least can outflank. Veteran squads with plasma/lascannons are cheap and provide good fire power. Platoons are trickier and more expense, but provide a ton of bodies.

Elites don't offer CSM much, but Fast attack has the vendetta or valkyrie, which is a really great flier.

Finally, you can take basilisks (or medusas) as a 1-3 squadron, so go nuts. As they are large blast, the BS3 isn't as big a deal, and I'd model them Iron Warriors if you want.