A properly matched ferrule has male and female counterparts of similar tapers. This resulting friction fit is secure, and it allows the load to be shared by maximum surface area. Ferrule connections with dissimilar tapers don't permit the same secure friction fit, and the load is carried on a much smaller area. Cutting the butt down will move the location of contact, but it won't change the taper, so I don't see that as a reliable fix. And keep in mind that the stiffer butt section you have won't likely flex at the same rate as the matching butt, which could create more stress on that small contact area.
Without having the blank sections in hand, no one will be able to tell you with certainty how secure the connection will be or how much it will affect the rod's action. Have you tried flexing the assembled blank? If so, do you notice movement or noise at the ferrule? If so, that may be a deal breaker for building up the rod as is (would be for me).
If it's worth the expense to you, and you want to preserve the length, someone could probably make a spigot ferrule for you (I assume yours is a tip-over ferrule). As for using the mid and tip sections only, I'd expect it would be a 3wt at best. You can use the common cents system to determine this beforehand.