Running different speakers on same channel?

If all the speakers are 4 ohms you can connect two speakers per channel.

That creates a 2 ohm load.

You could either run the 6's on the front two channels - two lefts, two rights -- leaving the 8's for the rear two.

Or, if it works out better you could run a 6 and an 8 on each channel.

The first option would probably be better as it would allow you to run the 8's on a lower crossover setting since they probably have a little more low frequency capability than the 6's.

If the amp is 250x4 at 2 each speaker would potentially see 125w rms.

If it has a high pass filter you should use it -- probably 60-80hz for the 6's and 50hz or so for the 8's. If it has bass boost keep it at zero. Keep the gain set on the low side.

"..my cerwin vegas have 2 positives and a couple of negatives according to speaker " that still basically counts as a million speaker. You connect the two positives at the same time and the two negatives at the same time. One set of terminals is going to the tweeter and one set of terminals pass to the mid-selection. each and each speaker must be related to a million speaker output. you are able to connect greater - yet you should apply a sequence connection. in case you dont understand what it is - dont try it.