come to my understanding that TC issues SEL and MEL rating which is valid for all types of aircrafts
Not quite.
There are ultimately nine different ratings available for persons who hold a licence:
i) All single-pilot non-high performance, single-engine land,
ii) All single-pilot non-high performance, single-engine sea,
iii) All single-pilot non-high performance, single-engine land and sea,
iv) All single-pilot non-high performance, single-engine and centre thrust multi-engine land,
v) All single-pilot non-high performance, single-engine and centre thrust multi-engine sea,
vi) All single-pilot non-high performance, single-engine and centre thrust multi-engine land and sea,
vii) All single-pilot non-high performance, single-engine and multi-engine land,
viii) All single-pilot non-high performance, single-engine and multi-engine land and sea,
ix) Individual Type Ratings.
I specify a licence because there are three other blanket ratings for those pilots who hold a recreational pilot permit.
I digress. As you can see, most general aviation aircraft will fit into one of the first eight ratings depending on engine and gear configuration.
The ninth - an individual type rating - is limited to those aircraft types that meet these requirements:
i) the minimum flight crew is two pilots,
ii) a cruise relief pilot, flight engineer, or second officer is required,
iii) All high-performance aircraft, or
iv) An aircraft that is not otherwise covered by a blanket type rating.
A Seneca does not require or meet any of those requirements and is therefore covered by the "
All Single Pilot Non-High Performance Multi-Engine Land" Blanket Type Rating. To obtain the blanket rating, you would complete the ride in any Non-High Performance Multi-Engine aircraft.
For the sake of the argument, you could complete your training in a Seneca and then go do the flight test in a Beech Baron. Why you would do that is beyond me though - you'd be doing the flight test in an airplane you're not familiar with and even though the blanket rating covers airplanes that fly the same and are not generally complex, they still have differences that you don't want to be thinking through in the middle of a flight test. Plus, I'm sure there is an insurance requirement somewhere that says you have to have a minimum number of hours in the aircraft before you do a flight test, especially if the examiner is not employed directly by the flight school (as was the case when I did my multi training).
What you could not do, however, is complete your training in a Seneca and then go do the flight test in, say, a King Air. The King Air requires an Individual Type Rating, so you'd have to do the training applicable to the type rating first, among other pre-requisites.