The RG lowers will fit but may look odd. There will be a gap between the bat wing fairing and the top of the lowers.
Better off using lowers from a bat wing bike
They'll fit. Just check pages 336 and 337 of the '12 P&A catalog. Better, confirm that the engine guard for the RG is the same for the EG--easy enough to do by visiting the parts counter at your HD parts counter.
Unless the RG's come with glove boxes, it's almost foolish to buy non-vented lowers for an '07 EG--buy color-matched vented lowers (or primered provided you have a great source for matching paint) and come out ahead in total cost, especially if you are thinking of adding CVO speaker boxes (which don't fit with non-vented lowers).
Will lowers for let's say a 98 Roadglide fit a 2007 Electra glide??? part #58841-98
The Road Glide lowers, made for '09 earlier Road Glides will fit any Electra Glide, Road King as well as the Road Glides. The only difference is the top of the RG lowers will have a flat top of the lowers. This is to fit the bottom of the RG Fairing. That will show when using the Bat Wing fairing (or no fairing with the Road King).
The Lower Speakers will fit the Electra Glide Pockets, but not the Road Glide Pockets.