Hi Benjamin.

I have experience with miRs, not with pri-miRs, whoever I recommend to use the taqman kit you mencioned it, it´s necessary since miRs are very small and by using an optimized kit your results will be more accurate than desing the primers yourself.  Also, if is a money issue I recomend to extract RNA using TRIzol method, the columns are very expensive and the yield and the amount is almost the same and the miRs are there, believe me, I change the method because the columns ARE EXPENSIVE. Also if you choose to buy the kit, used at half, for the qPCR, I used a total of 10ul per reaction, I think they recommend 20ul.

Hope this helps, cheers!

Yes it's perfectly fine. As a validation of your primers you could even knock down Microprocessor (ie Drosha or DGCR8) and should see an increase of pri-miRNA expression by qPCR. I did it many times and it works really well.

I´m also using primers designed by myself and they work perfectly fine.

About the TRIZOL for miRNas isolation we should be cautious..., I just came across with this retraction in Molecular Cell "http://www.cell.com/molecular-cell/fulltext/S1097-2765%2812%2900495-9", where authors state that "structured miRNAs with low GC content, such as miR-141, miR-29b, miR-21, miR-106b, miR-15a, and miR-34a, are selectively lost during sample preparation rather than degraded in the cell. The small RNA loss occurs when a small number of cells is used for RNA preparation using the standard TRIzol protocol (see Kim et al. [pp. 893–895] in this issue for details)"