Do certain string patterns affect query performance on a VARCHAR column?
Several points:

1) Store the data in two columns.

2) Normalize 'type' to a separate table, so you'll only put an INT into your index rather than a varchar(255).

3) Switch to UNSIGNED INT for the ids, and maybe TINYINT for the types.

4) Tell the app people to learn about multiple WHERE clauses for their SQL and give them a good index.

3) Once you do those, your question might become, "should my composite index be (id,type) or (type,id)?"