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The questions you are asking are deep into the realm of structural engineering. There are no "rules of thumb" to follow as every house is going to be different. The calculations involved in determining what is safe when you alter the supporting structure are very complex, and would require measuring and anylyzing what you have now, in order to predict what will work to prevent floor sagging, etc. I strongly recommend that you consult a licensed Structural Engineer to ensure the safety of your plan, and explain to you what you need to know.
Knowing nothing about your knowlegeable friend, I don't want to insult him/her, but I do want to caution you if you hear the phrase "over-engineer". Sometimes contractors who like to wing it will say things like this in hopes of saving some money. An engineer will design the beam to ensure safety for the expected load, plus some extra strength for a possible additional load (waterbed, hot tub, etc.) In Mass. this past winter, four roofs collapsed due to heavy snow loading. These catastrophes can happen when folks try a little too hard to save some money.
In an 80+ year old house even walls that once were not load-bearing can become so because of the way the house settled. Chances are good you may find supporting beams weakend by previous remodeling and/or wood rot.