That depends of what is your objective.

Search for literature about "microbial population structure in anaerobic digestion/biogas"

If you want to produce methane, then you need both of them, acidogenic and methanogenic. About the composition, are you asking about their quantity? if yes, this is still a difficult answer that several groups are trying to answer. Biomolecular techniques can somehow gives us an answer of the "microbial population structure in biogas reactors". If you want to produce hydrogen or acids, then is better to get rid of the methanogenic.

I agree with Juan, with the addition that besides the system's objective functions (methane, biohydrogen, valuable organic acids, oils, etc.) you should also take into consideration the nature of your fermentation substrate which could require a highly specific composition of your initial inocula.

Hi, this is a tricky question and in my opinion there is no straight answer.

In my research I have done metagenome analysis of sludge from several biogas reactors and generally the proportion of identified bacteria / methanogens is around 90-95% bacteria and 10-5% methanogens. However, in some metatranscriptome analysis we have seen that even though  methanogens are less abundant they are extremely active, representing around 25-30% of the active community. 

In my opinion it is not so important the proportion of bacteria/methanogens you have and what is important is that you operate the digester at the best optimal conditions for you specific inocula and substrate, and as Juan and Iulina point out depending on your goal you should adapt your community accordingly.

I hope this helps.

This article my help for your reference

Amani T, Nosrati M, Mousavi SM. 2011. Using enriched cultures for elevation of anaerobic syntrophic interactions between acetogens and methanogens in a high-load continuous digester. Bioresour. Technol. 102:3716–23.