preliminary results show that the battery's i have chosen have been doing very good actually capacity wise

1. Battery has 804mAh
2. Battery has 1409mAh
Totalling 2213mAh

Discharge of both in parallel showed a capacity of 2260mAh.

Now you might notice that the parallel capacity is higher but this is due to a miscalculation from my part.

Process has been done like this for 1. test
Discharge 1A to cutoff, then 0.2A to cutoff, this was also done for dual setup.

Next test will be done with 0.5A and 0.1A for individual battery's and then 1A and 0.2A for parallel, this will also reflect a better capacity for the battery's as individual.
1. battery has a total of 804mAh (tested) but this is not accurate as i have been discharging it at 1A (over 1C) so actual capacity will be higher.

But for 1. conclusion on making a dual (or more) battery with different cells will work, but my testing was done under stable condition's, real world will fluctuate depending on usage and in case of getting close to empty the battery controller in the device will properly shut down when seeing a sudden voltage drop, that said this will happen close to the combined capacity anyway so you will get 90%+ of the combined capacity.

Also note that the 1A drainage is higher then the N900 'should' use, and here we are talking about peaks, or your N900 will get VERY HOT.
Also phone battery measurement (capacity left) will be a bit screwy, the voltage meter here under discharge was fluctuating quite a bit, where normally it would just go down in a normal pattern.

So to sum up (yes i will shut up now ) using 2 different battery's will actually work, just please keep them kinda close (25% difference in capacity max).