After some time practising, my point of view as changed a little bit:
First I worked with a full French IGN map set, downloaded on bittorent. It's obsolete (2004), but quickly converted to OruxMaps. Of course, I got problem with the single map navigation, and these blank no map areas.
Few days ago I discovered Mobile Atlas Creator (MOBAC), with the possibility of viewing French IGN 1/25000 maps. It was so easy and fast to extract a large area (Alf North East Pyrenees). The result is, only one map of two layers 1/25000 and 1/50000 for only 180Mo.
So, considering that soon we'll all get a double 64Go microSD in our phones, we'll no more have to work with multi small filed maps.
So now, you'll have more time to improve the wms management
Mitch.