If Manifold 8 can open the XLS file in 32-bit, try 32-bit Radian as well. If the 32-bit Radian cannot open the file, then that's an issue, otherwise (if 32-bit Radian can open the file, but 64-bit cannot) it is probably just that you don't have the 64-bit Access Database Engine installed.
We need more details regarding DBF and SQL Server.
DBF: Could you post the DBF file that contains more than 2 rows but imports in Radian as only 2? If the data is confidential, contact tech support.
SQL Server: What specifically happens? You are connecting to SQL Server in Radian, open one of the tables and only get a couple of rows, but not all of them, correct? If so, is the table stored on SQL Server or is it some special table, like a view or, say, an ODBC fallthrough? What indexes does Radian detect (what's in the schema dialog)? Do you get all records if you try copying this table using SELECT INTO to a table in a MAP file (SELECT * INTO [tablecopy] FROM [sqlserver]::[table])?