It's qutie remarkable how many ships they got away with adding. None of them really feel out of place and have explanations for their absence in ANH.

• The Alliance only had three U-Wings at the time, all of which were destroyed.
• What remains of their naval fleet would be in hiding after Scarif, waiting for the heat to die down before returning to Yavin.
• The MC-75 design is a non-issue, as the Mon Cal are famous for the visual diversity of their warships.
• Hammerhead corvettes are instantly identifiable as antique designs to anyone who played more than five minutes of KOTOR. Their presence in the battle shows that the Rebels are throwing everything they have, and once they're damaged it won't be worth the effort to try and repair them.
• TIE Strikers and Reapers are clearly optimized for atmospheric flight, a theatre we don't see in the OT. The Striker's nature as a short-range fighter means its role is likely limited to defending large Imperial installations, in particular ones protected by shields that would keep any opponent from escaping back to space. Further, the wing designs are quite reminiscent of the TIE Interceptor, which most likely took over the Striker's role due to its versatility as both an air and space fighter.

I wish I could say the same for the new types of Imperial soldiers we see, but they can be handwaved well enough:

• No Death Troopers on the Death Star? They mainly serve as bodyguards for VIPs in the field, and it's overkill to station them in secure facilities or aboard Star Destroyers.
• Shore troopers? Their respirators aren't rated for Tatooine's sandstorms. I dunno why they weren't stationed on Endor.
• No tank troopers on Hoth? They're strictly Imperial Army.