Can Egg Heist be played solo or with a partner in Aniimo?

Solo is not a stripped-down fallback here. Aniimo has a full single-player mode and the majority of its content can be played entirely alone, so a run with no partner is a normal way to play rather than a concession.

Bringing a friend changes one thing you should sort out before you start. In cooperative play each player takes only two Aniimo into battle, and the game pulls them automatically from your first two roster slots. There is no team select at the door. Reorder your roster first, or you go in with whatever happened to be sitting at the top.

Two is also the ceiling for this one. Cooperative parties elsewhere in Aniimo hold up to four players, so if you are used to running a full group for other shared activities, expect a smaller team here.

Getting a friend into your run

You cannot drop into someone's game uninvited. Joining another player's instance requires an invite from that player, so one of you hosts and the other accepts.

Platform is less of a barrier than it might be. Aniimo supports cross-platform play, including players on PlayStation 5 and Xbox together. The cost to watch for is on console, where online multiplayer may require a paid platform subscription. That applies to the co-op half only. A solo run needs none of it.

Not settled: The solo and two-player options described here are the rules for Egg Heist's PvE mode specifically.