well i have a lot of experience in hip hop recording (3500 songs

and counting). i pretty much only record hip hop and r&b on a regular basis 50 hrs a week.
i now have a template that i use for hip hop. the most important thing about making the vocals sit in with the beat is compression. there is literally no dynamics when it comes to hip hop. since the beat is an industry instrumental (im assuming since you said it was mixtape) the beat is already mixed well (no eqing required). HEAVY compression i mean HEAVY! up to 15 decibels of gain reduction (usually my threshold is set at -30db with a gain make up of 19 to 19) at times with a 4:1 ratio, very fast reaction (less then 10 ms) time with a medium to fast release time. put this on each vocal track (even the add libs). turn the addlibs down about 6 decibels less then the main track. this makes it fit inside the beat because its usually the peaks that make it feel on top of it. i buss all of the vocals together and depending on the person, sometimes i need to add a little gain to 8k and above to add presence. if you want you can check out my myspace for a quick example of what ive done ("shut yo mouth" -quick 5 minute mix)
if all else fails send it to me ill create a very clean mix if it was tracked out well
good luck!
-Lee
EDIT: i almost forgot. hip hop vocals are usually pretty dry, but if you add a VERY little amount (at least 30 decibels below the main level of the vocals, you should almost no be able to hear it at all) of delay set at 300ms or so with no feedback it brings the vocals forward without making them sit on top of the beat too much. if you had reverb, use it very very lightly and with a fast decay (small room).