There isn't going to be any difference in the amount of metal left on your table from tacks. You might try tacking a tab on the part and clamping it to the table but having to clean the table occasionally is a part of the job. Keep your welds in a horizontal position by repositioning the tee. Do approximately 1/3rd of each joint at a time to equalize and minimize the warpage. Undercut describes the condition where the weld metal does not roll into the edge, it rolls under leaving the edge of the part visible. MIG is easier to handle and the efficiency of not having to chip slag and stop and pick up a new rod lowers the time to weld and the cost. In some cases, like outdoors or field welding, a MIG will not provide as good of weld as a stick since wind will reduce the flux effect of the cover gas where as a stick is unaffected. As far as tips, the first rule is always work safely. Check and recheck tacked up assemblies are correct before you start to weld. Use common sense and build your experience in how to minimize warpage by working in different areas instead of one end to the other. Get an AWS inspection and testing requirements for certification. If you enjoy welding, have pride in your work.