no, I mean you can play with an army that consists of multiple different races as long as their point value doesn't excede the agreed apon amount, and as lond as your buddies dont mind you playing what amounts to 2 half sized armies.
The way my friends and I play, is that you can have allied units as long as it MAKES SENSE, allying Imperial Guard with Tyranids wouldnt happen, but it could with Marines, Tau, even Orks and Chaos (Fallen Marines and the humans that follow)
My army will ally itself with anyone that isnt totally evil (because they're mercenaries and all), anyone evil being Dark Eldar, Tyranids, Chaos, and Necrons.
Here's a good chart of alliances that make sense except under the most extreme circonstances.
Space Marines: Imperial Guard, Sisters of Battle, Eldar, Tau
Imperial Guard: Space Marines, Sisters of Battle, Tau, Eldar, Chaos, Orks
Sisters of Battle: Imperial Guard, Space Marines, Eldar, Tau
Orks: Imperial Guard, Dark Eldar, Chaos
Chaos: Orks, Imperial Guard, Dark Eldar
Eldar: Space Marines, Imperial Guard, Sisters of Battle, Tau
Dark Eldar: Orks, Chaos
Tyranids: Themselves 😃
Necrons: Themselves 😃
Tau: Imperial guard, Space Marines, Sisters of Battle, Eldar, Kroot (well duh 😃)