In what order do you recommend leveling them up?
		
		
	 
I think the hardest characters to welcome are Donald, Zurg, and Mother Gothel so putting the priority on trying to improve those welcoming experiences I'd say that it is important to get:
Pluto, Daisy, and Minnie to level 10 to get the most towards your Donald experience
Rex to level 10 and Randall to level 10 to get the most towards your Zurg experience
Maximus, Prince Phillip, Flynn or Rapunzel, and Randall/Sulley to level 10 to get the most towards your Mother Gothel Experience
The Fairies, Flynn, and Mickey were key for Prince Phillip tokens and the Fairies helped a lot with getting each others tokens
Pete helps a fair amount with Mother Gothel's daggers and other later characters so getting him to level 10 is helpful 
Some characters that pretty much became useless towards the later more difficult welcoming tasks were Roz, Bo Peep, Boo, Celia, Woody, Sarge, Hamm, and Wall-E     They were useful though with getting Monster Inc, Toy Story, and M&F characters to level 10 so they were still important to level up so they could help level up characters faster that would be needed for the harder welcome experiences later in the game.
For me I have never had my castle dormant.  There has always been some character in there upgrading.  I started out trying to get all of my M&F characters to 10 then all of my Toy Story characters to 10, but you can only upgrade one character at a time and eventually I fell behind and wasn't able to finish getting my Monster Inc characters to 10 before it became more important to start leveling up Tangled and Sleeping Beauty characters to level 10 to help with welcoming the harder/later characters coming into the game.   I'll go back and finish the last level on my Monster Inc characters (Roz, Sulley, Boo, and Randall as well as Eve and Wall-E) later when I have nothing else to do.  My priority now is finishing the later groups like Tangled and Sleeping Beauty characters since I would assume they will be more useful with helping to welcome the next round of new characters into the game.  I'd also like to keep the Incredible characters upgrading on a good pace as I go.
Currently the players I have at level 10 are:
Mikey, Pluto, Minnie, Goofy, Daisy, Donald (in 16 hours - he is currently upgrading), Pete
Jessie, Woody, Buzz, Bo Peep, Hamm, Sarge, Rex
Mike, Celia
Rapunzel, Maximus
Tinker
Prince Phillip
Fauna, Merryweather
Zurg, Aurora, Flynn, Flora, Wall-E, Eve, Randall, Roz, Boo, Sulley, and Mrs I all have the tokens to go to level 10 I just haven't had time yet.   
I'm working on getting Mother Gothel's last round of tokens so she'll be able to go to level 10
Dash is level 8 ready to go to level 9
Frozone is level 7 ready to go to level 8