Senegal has been a destination for organized bicycle tours for decades now. As to whether it is safe, that depends on what kind of tourist you are. Some people can keep their wits about them in Africa, while others fall right into scam or robbery attempts. Of course, if you are on a tour you’d be insulated from that (but at the cost of being insulated from the local culture too.) Among the journals at the famous Crazy Guy on a Bike site, you’ll find quite a few on travelling in these two countries.
As to whether those agencies you link to are a good bet, this probably isn’t the best place to ask. Lonely Planet is a publisher for independent travellers, and the Thorn Tree is populated mainly by those that dislike organized tours. You’d have to Google around to find reports of which agency is best.