Legumes are plants capable of fixating their own nitrogen, thanks to rhizobia bacteria found in the roots. Inoculants help promote that bacterial development from seed. The higher nitrogen content found in legumes because of this unique ability makes them have comparatively higher protein than most other forages. If you plant legumes, inoculate the seeds, and don't later fertilize with nitrogen (use an 0-#-# fertilizer). When you add nitrogen, the bacteria and the plants will use the nitrogen you supplied (at a cost) instead of fixating their own.

Also, keep in mind the type of fertilizer, form, application method, and plant availability can make a difference. This shouldn't be a real issue for you, since this issue is really more geared to the nitrogen than the phosphorus or potassium.

My suggestion is keep it simple. I've been seeing a lot of people lately overdoing it lately. Instead of taking the best one or two forages they can grow and focusing on maximizing the the potential of only those one or two forages, they try to plant a wide variety of species and end up with marginal production of nearly everything. It may take some experimenting, but focus on what you can grow and grow well.