Agreed ^^ I can almost guarantee you're not gonna lose one subscriber over something like a guest strip. People do them all the time, and readers love it the same reason you do; you get to see your comic done in a different style than your own, with different ideas that are fun for the audience to imagine.
I've put guest strips into Time Gate and as far as I know, my readers have never had a problem with it.
Making a new series, TBH, should probably only be done if you really have a lot of guest strips and are planning on releasing new guest strips in the future when they become available (and really not even then TBH because new "series" should be used when you're creating a new series. Not just a set of alternate episodes for what's essentially a series that's already available). Other than that, I'd put them in with your current comic, let people know what's up, and they'll understand. Don't assume your readers will leave you at the drop of a hat
You'll be fine putting it into the main series as specials, either as separate episodes or at the end of current ones, whichever you choose. Also remember that you won't carry all your current subscribers over with you to the guest comic series if you choose to do that, and chances are the guest strip series will never be on par with the main series in terms of subscribers/dedicated readers, so in that case, it's probably not even worth it to divide your fanbase up like that y'know?