Making a list for how Trixie and Rarity are compatible romantically with Twilight and why.
Trixie's compatibility with Twilight
>Life long rivals - Pro & Con (that's a wash)
On the one hand it can lead to some kinky hate sex on the other hand they or at least Trixie still has the urge to one up each other.
>Both are heavily into magic - Pro
They have that in common
>Trixie's musk - Pro
>Trixie's passion doesn't make her much money - Con
>Twilight and Trixie had a child with each other (even if Luna and Rarity are technically involved) - Pro
>Trixie beats Twilight ass sometimes and Twilight isn't even into it like Sonic is with Bartleby - Con
So that's 3 pros and 2 cons for Twilight being with Trixie
Rarity compatibility with Twilight
>Best friends - Pro
>Rarity can be relied on to have Twilight's back 90% of the time - Pro
Thats 2 pros I can think of and 0 cons I can think of.