Yep, "Marnie finds stray pets that you can adopt" from the mod description. You should be able to
adopt from Marnie's, and yeah, you can have multiple cats/dogs/horses. There's a lot of information on the mod page, which can be a little overwhelming, but it's because it does a lot of stuff :) For example, in the configuration options section, it says you can control what the chances are of finding a stray at Marnie's:
StrayChancePercentage : The chance of a stray pet appearing each day at Marnie's, after the player obtains their first pet. By default, this is set to 60%
Good luck, and don't forget to use the
beta version as the mod page says!