I believe you have the wrong approach.
You need to get informed about the piece of software, its scope, its functionality, limitations and so on.
You then need to try and use it and see if it suits you and your needs.
My personal opinion about Mullvad web browser has limited value; yet the fact that I took the time and willingness to create and participate in this thread is an hint my personal opinion is rather positive.
Your opinions and questions would be better answered, as a start, by you yourself getting informed and using the software and NOT by a random stranger on a forum stating "it is a great piece of software you must use it blindly".
By the way, to answer your main question: despite of I have not tried, you can use and install other VPNs and use them in combination with Mullvad web browser without particularly impairing its fingerprinting and privacy protections, provided that you stick with what the software developer recommends which is to use Mullvad web browser "as it is" without changing ANYTHING after its installation, apart from the security level safe-safer-safest.
You can install other add-ons if you wish but it is not recommended because it can impair its fingerprinting protection feature.
You can change the browser settings, ublock origin settings and lists, noscript settings and so on, if you wish but it is not
recommended because it can impair its fingerprinting protection feature.
You can use almost any VPNs without the need to install its web browser add-on in order to manage it but can rely on the software client side, either its official client or by downloading and using wireguard/openvpn with the configuration given by the VPN provider; with the majority of VPN providers there is no need (it is not mandatory) to install the web browser add-on in order to surf the internet behind that VPN.
All this info can be found online, first place to look at is the how-to sections of the websites related to the VPN provider.
I believe and hope there should not be anything left to discuss if you <read this post from the top>.