While allies and slivers sound alike, and play similarly, they have a few differences. Slivers is more about one creature gives the rest something when on the field, while allies get triggers during the summoning of a ally. They also get focus more on boosting themselves.
For instance, many cards in the deck have the ability of when i summon another ally, it gets a +1/+1 counter, also, one variant is a flying blue three drop while another is a white one drop. Other effects include mill for my number of allies every time I summon an ally, and gain protection from a color of my choice until end of turn each time I play one, and even gain 1 life for each ally every time I play one. It is slightly less offensive but if you build up an army it becomes a major threat.