Yes of course. They're monsters with levels, just like normal and effect monsters.
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you cant
No, because it wasn't 'properly' special summoned. To be able to special summon a Ritual Monster from the graveyard, it has to be initially summoned by Ritual Summon. Ritual Foregone does not Ritual Summon.
You have five monster slots on your Field, so even a card that can summon an unspecified amount of monsters (like Return from the Different Dimension) you will still only be able to summon a maximum of 5 to your own side of the field. There are no cards that can increase this, and Trap Monsters take up a monster card slot so it's not like you could have five regular monsters and an Embodiment of Apophis.
A tribute summon is a form of Normal Summon. So that means you can only tribute summon OR normal summon ONCE per turn.
Joey's opponent sacrifices Fenrir and Inpachi to Tribute Summon Maju Garzett whose ATK value becomes 3000 with 0 DEF.