No, Muscle Tools and X-Muscle System are not the same products. They're different projects and they're not even compatible.

All skeleton models are included under Royalty Free License

Yes, for upgrading the add-on contact X-Muscle System support and Blender Market team.

X-Muscle System customers will be supported for the whole product life-time with all the documentation, video-tutorials, software updates, new features and bug fixes. All future updates and new content are free.

X-Muscle System is advanced system for creating character's musculature. It includes multiple automation tools, which can not be find in Muscle Tools. Making the rigging time down from days to minutes allows to focus only on the creative part of the work. The most important exclusive features X-Muscle System provides:

• Advanced and intelligent X-Mirror
• autobind feature
• auto-weight paint feature
• powerful renaming system
• multiple muscles control system 
• easy switch from flexor to extensor muscle type
• intelligent dynamics cache update
• intelligent Vertex-Groups Manager
• intelligent modifiers and drivers management
• designed for freestyle workflow (symmetrical pose is not required)
• micro skin Controllers which improves deformation quality and precision
• supports mesh models with shape-keys and drivers
• muscle pinning with x-mirror support
• anatomy bundle included

Each new version is available for download on Blender Market. Old files are always replaced by new ones. To download the update all you have to do is to download the same files. Add-on as well as documentation are always versioned so if you have any doubts just look for the version numbers in downloaded files.

In this case please contact Blender Market support. They will help you and instruct step by step how to download the updated files.

Each model has Subsurf modifier enabled by default. Disable the Subsurf for the Viewport in Modifier's section or use the Simplify in the Scene section to simplify the whole scene.

Some control bones transformation may be limited by default. If you want more elasticity of your model just unlock transform for Loc Rot or Scale for particular control bones.

X-Muscle System XL variant is deluxe edition of the add-on including high quality Anatomy Skeleton Bundle rigged models and Exclusive Early Access to Beta Features.

Current Anatomy Skeleton Bundle content:

• cat
• dog
• horse
• human male V1 and V2
• carnivore dinsaur
• human female

Standard add-on version includes one rigged model - human male V1 skeleton

At the moment there are three ways of exporting character's mesh with muscle deformations. First of them is NewTek MDD exporter, which will export shape-keyed animation keyframes. The second one is done via Corrective shape keys. This second method allows to control specific muscle behavior within game engine via bone-driven shape-keys. Both add-ons are provided with blender and need activation in User Preferences. Third one is done by baking displacement map.

Also Alembic with its pros and cons can be used.

Starting with 2023 X-Muscle System support for older Blender versions than 3.X has been dropped. The add-on gets support for its product life-time and the latest Blender it supports is 4.X.