I guess you could use any as long as it is good and tasty, mild would be no good as you wouldn't taste it. I still prefer mature myself but then I am mature myself!
Leicester will be too mild but will add lots of colour! Although if you would otherwise be throwing it out you may as well use it but I would add parmesan to it for taste.
I always get the mature cheddar as it has the most taste and therefore you need to use less of it for flavour, less fat/calories and tends to be about the same cost as a cheaper less tasty one that you have to use a lot of. Also, I hate dishes that are swimming in fat. Ugh!