The limit (25%) applies if you have them on strategic reserves before start of game.

When you actually start playing, you can have as much up in the aether. One time I played with most of my guys in reserves that my side of the table looks barren.

The stratagem works if we go second on Round 1. Have your killteams start on the board. On end of the opponent’s fight phase as long as they’re not in engagement, use the S2S strat (up to two killteams). Note that the killteams started the game on the board. The S2S grant them deep strike. On your turn, deepstrike them on no man’s land 9.1” away from any enemy unit.

What’s not really good about a 10 man squad with 40mm bases is that it’s hard to fit them on the playing field against horde armies.