Guard + Marine Apocalypse will give you the chance to represent Space Marine Serfs. Hordes of IG in rhinos and razorbacks, backed up by predators, vindicators, razorbacks, etc. Command elements of real marines and terminators. Could be cool.
I love Tyranids, but it's really difficult to wrap my head around Space Marines palling around with them. In the old Codex, there's fluff depicting an Imperial scientist about to be sentenced to death for merely suggesting that there's much to be learned from Tyranids, so fighting alongside them really blows my mind. IG makes the most sense, although the Imperium has (very rarely, and very briefly) made use of Orkish allies to fend off an immediate threat.
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