It's possible to insert into variable record from table even if amount of fields in the row are different. But there is an error on select variable after insert from a table with "different row type".

Consider next queries:

CREATE TABLE tab (id int, t varchar);
INSERT INTO tab VALUES (0, 'str00');

SELECT pgv_insert('vars', 'r1', row(1, 'str1', 'str2'));
SELECT pgv_insert('vars', 'r0', tab) FROM tab;
SELECT pgv_insert('vars', 'r1', tab) FROM tab;

SELECT pgv_select('vars', 'r1');

The last one statement of pgv_select bring an error like:

   pgv_select    
-----------------
 (1,str1,str2)
 (0,"\rstr00~",)
(2 rows)

The output may differs, but apparently there is a problem with memory access here.

A more general question is it should be possible to insert record with different amount of fields in the existing variable? Probably not, since behavior becomes too implicit.