Ok so I should clarify that I haven't actually had opportunity to use my sport with unity capsule yet coz bubby still hasn't made her appearance, but I've had a good play around with mine so I can say....

Yes you can definitely use the toddler seat underneath with capsule ontop. The adapter just clips in where the bar or toddler seat would go on the front. And can just stay there to put in car or whatever. The top seat (on mine at least) can't be FULLY upright with the capsule but it pretty much is I think I just loosened the zips a little.
FWIW i got my capsule second hand in as new condition only 6monthd past manufacture date for $140, hire ones are used anyway it just depends how much you wanna trust the person selling I guess. I also kept an eye on gumtree and managed to get the adapter for $15 as opposed to $40 new.

Oh and my sil has used the unity with her sport for her youngest two and loved it! Unfortunately her baby didn't love the capsule at all so it didn't get utilized as much as expected.