Good news! You can record every bike ride to either Strava or RideWithGPS (or both!) for free and then Bike Mec will sync your rides to the app and keep up with all your service schedules.

Absolutely! Any bicycle computer which is capable of syncing your activity to either Strava or RideWithGPS after you finish a ride will work with the app. Bike Mec receives a notification almost immediately from either service once the ride is uploaded to tell it to update your maintenance schedule. You can also use the Strava or RideWithGPS apps to record your rides but them you must remember to log every ride you do in those apps. A bicycle computer is definitely the easiest way to go.

Without a service keeping track of each of your rides, Bike Mec doesn't know the latest rides, distance, or moving time for each bike. So for Offline bikes service can only be scheduled by the calendar.

Many riders double post their rides to both Strava and RideWithGPS. Why not: it's free, protects you from losing your data, and each site has it's own strengths. But if you happen to sync Bike Mec to both services you probably only want to see one of each bike you own. ;)
Hiding a bike in Settings also lets you ignore older bikes which you may no longer be riding but are still in your ride history.

If you create a bike ride on Strava or RideWithGPS and haven't gotten around to configuring your gear then Bike Mec has your back. Those rides will be logged for the Default Bike and if you only have one bike then maybe that's all you'll ever need. If you want to schedule service on multiple bikes though then you'll need to set those up in Strava or RideWithGPS and make sure each ride is logged for the appropriate bike.

In order to get the complete history of all your bicycles, Bike Mec downloads all of your historical activities from Strava or RideWithGPS. With all that data Bike Mec then calculates the number of rides for each bike, the moving time, and distance covered. Once the initial download is complete Bike Mec only has to sync rides that have happened since the prior sync which is much quicker.