I understand, but that's not always going to work, as not all databases support the same types. The more databases you want to support, the more you run the risk that you have to use a typeconverter here and there to make the types match.
Although you want to keep just 1 db for designing, it will likely be a bit of a problem in the end, unless you use just a small subset of the sqlserver types, namely the ones which are available in all databases you support and which also have the same .net type.
The converter can't always make the right decision based on the information it has. Therefore it's often better to do:
- convert project to target db into new lgp
- generate DDL SQL using template
- check DDL SQL generated and update it to match what you want
- create schema in db from DDL SQL
- refresh the new lgp file onto the schema created.
From there, you probably will have to maintain 2 or more database schemas as it's unavoidable that you will run into type mismatches here and there, or sillyness like the '-' characters in names, or other things, like max length of a name, max length of a field which isn't possible on db 1 but is possible on db 2 etc.