Tire Replacement

OK the time is growing near where I think I need a new tire. My front is fine but my rear has developed a "flat spot" in the middle. What my argument is here is that the lean angles or sides of the back tire are in pretty good shape.

(I believe this flat spot was caused for when I was stuck in traffic last summer on an 80 degree day on freshly laid blacktop. The blacktop MELTED into my tires and left hundreds of pebbles and rocks embedded in the rubber. I couldn't pick them out by hand so........I did a rather long [10second] burnout to get them out; all the while thinking "Man im f***ing up an expensive tire right now." Either way I think this may be part of the reason behind the flat spot in the tread).

Nevertheless I included some pics of BOTH my tires. I would appreciate if you guys could tell me if they NEED replacement or how soon I should need to. My biggest concern here is I am so unbelieveably broke I cannot quite afford a new tire right now. Don't worry I won't ride on a bad tire; this would just mean the bike would take of a week or so until I can afford a new one :-|

Also I was looking into a different tire. I'm thinking of getting a Dunlop D208 180/55 x 17 as opposed to the previous Dunlop Sportmax. And this brings my next question. Due to the fact my front tire is in fine shape.......would it be a problem mismatching tires by buying a different name rear than front? If this is something dangerous or just "wrong" to do I would rather get an OEM tire (again due to my broke college ***).

Rear Tire 1
http://img134.echo.cx/my.php?image=tire16ue.jpg

Rear Tire 2
http://img134.echo.cx/my.php?image=tire20gu.jpg

Front Tire
http://img134.echo.cx/my.php?image=fronttire4qc.jpg