Replacement effects that modify creatures of a certain type as they enter will apply after you apply Rin and Seri, Inseparabler’s first and second abilities, as appropriate. For example, if you control Rin and Seri, Inseparabler as well as a permanent with “Each Dog you control enters with an additional +1/+1 counter on it” and you create a 1/1 Cat token, that Cat token will be a Dog Cat token and will enter with a +1/+1 counter on it, a woof, and a purr.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.