I promise you that the poster above knows what a Mantis is
What species are you hoping to keep? 40 gallons is about the minimum for a large Peacock mantis (
Odontodactylus scyllarus), but on the larger side for some of the other, smaller species, which isn't a bad thing.
Having kept one myself, I have some concern about your plan to keep two in the same tank, I think that often ends in one very fat Mantis
Likewise, I'm not sure how successful keeping Clownfish will be, though it might work. I know some have decent success keeping small, savvy and quick Damsel fish with a Mantis, so the closely related Clownfish may work out
I disagree about the Clean Up Crew. Yes, the Mantis will go after them, but slowly over time as they see fit. This is healthy for a Mantis' well being, as they can naturally stalk and prey on the CUC critters when the mood strikes them. CUC animals are relatively cheap to replace, and should be a part of any Mantis setup, in my opinion. Take a look at places Like ReefCleaners.org for cheap CUC packages, or I personally recommend
CoralMonsters.com.
I personally prefer the Fluval AquaClear filters to the SeaChem Tidal filters, but the Tidals are popular. I'd go larger than the 55, I'd get the 75 or even 110 for increased flow.
I would skip the Sicce skimmer for now. You might not need it, and I don't believe that is a very well regarded choice for a skimmer.
@Stomatopods17 will know all this for sure, if they have a minute to reply.
I hope this helps, good luck, and keep us posted on how it goes